The present invention relates to an underground pipe, specifically, an underground pipe made of a synthetic resin material and which is intended for use as a water main pipe, sewer pipe, sheath for protecting electrical cables, or the like.
A prior art pipe of the same general type to which the present invention pertains has been disclosed by the present applicant in Japanese Laid-Open Utility Model applications Nos. 44582/83 and 44583/83. In the former application, there is disclosed a pipe having a reinforcement constituted by an integral helical U-shaped rib. The open side of the rib is closed with a band-like member so that the inner surface of the pipe is continuous. In the latter application, the reinforcement is constituted by a helical U-shaped rib fused to the outer surface of a continuous pipe.
Both types of pipe have a generally high strength, and are advantageous in that respect. Moreover, these pipes have a good pressure-withstanding ability and are light in weight and easy to handle. On the other hand, in both cases, the load is concentrated in wall portions opposite the reinforcement, making it necessary to increase the wall thickness in these areas. This calls for an increase in the amount of resin material needed to form the pipes and makes them heavier than is desired.